Biological consequences of ionizing radiation primarily arise when energy deposited within cell nuclei ultimately affects the DNA. Initially, the deposited energy creates ionized species at random. If ionization occurs away from the DNA, the radical ions may diffuse to and react with the DNA; if it occurs within a chromosome, the DNA is affected directly by the ionization. Electron-loss and electron-gain centers formed directly within DNA, molecular free-radical intermediates, are an important link connecting the initial event of energy deposition and the biological endpoints. This is an important health-related issue as direct ionization of DNA and its hydration shell is estimated to cause about half the cellular effects. Consequently, it is important to arrive at unambiguous identification of DNA radicals stabilized sufficiently long to undergo subsequent chemical reactions, and to understand the possible reactions in which these radicals may participate as may be dictated by their immediate surroundings. Thus, long-range objectives of this project are to identify unambiguously the possible radical products of directly ionized DNA, the factors which can control their stabilization, and the ways in which they may transform or react chemically within their surrounding. To meet the long-range objectives, specific aims for this project are: (1) to refine and further develop models describing the role of molecule-molecule associations, such as hydrogen bonding, in controlling the stabilization of DNA radical products; (2) to identify the mechanisms in which radicals from direct ionization may lead to strand breaks; (3) to further develop techniques for using oligonucleotides to extend these studies to well-defined, DNA-like systems. The approach is to use crystals of selected molecular systems and to apply the high-resolution, radical-specific, methods of electron paramagnetic resonance spectroscopy (EPR) with its companion, electron-nuclear double resonance (ENDOR) spectroscopy. Use of crystals permits taking advantage of the full, atomic-coordinate-level, characterization of these systems available from diffraction studies. To meet the aims, spectroscopic parameters and radical yields will be measured in a selected set of model systems containing hydrogen-bonding and other molecular associations reasonably like those in DNA. From this information and details of the crystal structure, patterns of behavior will be sought and related to specific elements of molecular associations. Final transfer of the results to DNA will be made by carefully selecting and employing the well-defined, DNA-like, systems provided by oligonucleotides.